Monthly Archives: January 2020

Philadelphia Weather Outlook

WEEKEND WEATHER OUTLOOK

High pressure dominates the east coast today.  Despite the very cold temperatures yesterday and today, a moderating trend will develop by the end of the week.  This high pressure system over us will move off the coast, but it will be blocking any storms from affecting us through Friday.

This Saturday, a coastal low pressure  system is expected to develop around Delaware Bay Friday night and move slowly northeastward through the day Saturday as an upper air low catches up with the surface low. The models are in remarkable agreement about this system right now.

Ordinarily, this would be a snowstorm for us but not this time.  What’s lacking, unsurprisingly for this winter, is a cold high pressure system to the immediate northwest.

GFS ensemble Precip rate  forecast for 7 PM Saturday.  White line is 32º surface temp line.

So Saturday will be rainy for most of the day, with QPF values near 1.00 inches water.

As the upper low moves into position, some cold air may be enough to provide some snow flurries late Saturday night, but temperatures are expected to remain above freezing.

Windy for Sunday!

 

Winter Weather

SNOW/SLEET UPDATE

2 Comments

Update Sat 03:35 PM — The changeover to sleet has occurred  in the time frame predicted by the NAM model.

The 1PM run of the NAM, which just became available, predicts some freezing rain here between 5-6 PM.

NAM 18Z model forecast for 6 PM-   Freezing rain in red, sleet -pink,  green rain

Update Sat 12:14 PM — Latest Canadian Global, just available, is very similar to the  lower accumulations of the NAM.

CMC GDPS Model Snow accumulation totals 7 PM

Update Sat 11:55 AM — Earlier, I said I was going with the NAM model’s lower accumulations, but the latest GFS just available still insists on more.  It has been very consistent and it’s something that needs to be considered as a possibility.

Latest GFS model continues with higher snow totals at 7 PM.    Click for larger image.

Update Sat 09:04 AM — NAM data just became available.  I think we’re going to go with these much lower snow/sleet totals—

Last night’s models have continued the dichotomy between the global models and the short range high resolution models.

The short range (NAM, NAM NEST) have further reduced accumulations.  Here’s the latest NAM snow/sleet depth for 7 PM tonight—

NAM Snow/sleet depth at 7 PM Saturday.  Click for larger image

These models continue with a warming above 6000 feet, changing the precip to sleet by 3-4 PM.

The global models (GFS, CMC, ECMWF) continue with lower level cold and dynamic cooling from heavier precip.  They maintain snow levels almost 1 inch over the above totals.

Just as last night, I’m leaning towards the NAM and the lower snow/sleet totals shown above. I’m assuming the GFS is over-doing the cold air and precip.